Thermoplastic starch compositions are often used to make final plastic products. The thermoplastic starch composition is heated and formed into the products using known techniques such as injection moulding, extrusion blow moulding, thermoforming, injection stretch blow moulding, cast film extrusion, blown film extrusion and fiber spinning/extrusion. Often the thermoplastic starch compositions have many of the characteristics of other plastic compositions, such as polyethylene or polypropylene, however, they have the added advantage that they can be sourced from renewable resources and, are often readily biodegradable. This provides for more environmentally friendly plastic products.
However, a problem with thermoplastic starch compositions, is the tendency for discoloration upon heating. There are various steps and processes in the formation and processing of the thermoplastic starch composition that require it to be heated. Examples include, destructuring of the starch, and heating of the thermoplastic starch composition during product forming processes such as injection molding, extrusion blow moulding, thermoforming, injection stretch blow moulding, cast film extrusion, blown film extrusion and fiber spinning/extrusion. Discoloration can be further increased when thermoplastic starch is blended with other thermoplastic materials or fillers, especially when high thermal and mechanical energy processes such as compounding are used. Discoloration can be seen as a yellow or brown tint, which is viewed as an undesirable characteristic of the end use product by the consumer. In its most extreme form, discoloration can result in black material.
Discoloration can also occur in the final product if it is subjected to direct heat or UV, such as being left in direct sunlight.
The discoloration of thermoplastic starch can be attributed to numerous complex and interconnected non-enzymatic ‘browning reactions’. ‘Maillard-type’ reactions are one type of non-enzymatic browning reaction. These reactions occur between residual amino compounds and reducing groups present in the starch compounds. This reaction occurs at intermediate temperatures and in the presence of moisture. Maillard reactions are known to be inhibited by compounds such as sulfite (sodium sulfite, sodium bisulfite, and sodium metabisulfite). Oxidation is another type of browning reaction that occurs at elevated temperatures in oxygen rich environments. Caramelization reactions are yet another non-enzymatic browning reaction that occurs in starch and other polysaccharides. Caramelization reactions are complex reactions that occur under elevated temperatures and low moisture content. Caramelization reactions are catalyzed by numerous agents and the type of caramel produced is characterized by the catalyst used. For example, type I caramels are produced under basic conditions and type II caramels are produced under basic conditions with a sulfite compound such as sodium sulfite.
Therefore, a compound such as a sulfite could lower discoloration associated with the Maillard reactions, but increase discoloration associated with Caramelization reactions, especially under conditions of high temperature and low moisture.
Caramelization reactions can be inhibited by low pH conditions. However, low pH conditions catalyze starch hydrolysis, which leads to a reduction in starch molecular weight. A reduction in starch molecular weight can result in a decrease in viscosity of the molten thermoplastic starch. This may negatively affect certain polymer processing operations such as injection stretch blow molding, extrusion blow moulding, cast film extrusion, and blown film extrusion as the low viscosity thermoplastic starch composition cannot follow the contours of the mold and so results in excessively thick and/or thin areas of the final product. It may also negatively affect the mechanical properties of the final product, such as tensile strength.
WO2008014573 discloses the use of the reducing agents sodium sulfite, sodium bisulfite and metabisulfite as anti-discoloration agents for thermoplastic starch. However, as mentioned above, sodium sulfite and sodium bisulfite are known to catalyze the formation of type II caramels especially under basic conditions, elevated temperatures, and lower moisture content. It has been found that under the elevated temperatures, high energy, and low moisture processing associated with thermoplastic starch production, these agents are not sufficient to reduce discoloration and actually contribute themselves to additional browning under certain conditions.
There is a need in the art for thermoplastic starch compositions exhibiting less discoloration during processing and in the final plastic product, than is seen using known thermoplastic starch compositions.
There is a further need for thermoplastic starch compositions that exhibit less discoloration yet also maintain a desirable molecular weight starch during processing.
It was surprisingly found that the thermoplastic starch compositions of the present invention resulted in the production of thermoplastic starch materials exhibiting less discoloration than is seen using known thermoplastic starch compositions. It was also surprisingly found that the thermoplastic starch compositions of the present invention, following extensive thermal/mechanical processing, comprised starch of a higher molecular weight, than is seen in other thermoplastic starch compositions subjected to the same processing.